The present invention relates to the field of piston rings. More particularly, the invention relates to the field of materials used to manufacture piston rings for automobiles.
Piston rings that are used in the automobile industry are commonly nitrided by subjecting the piston rings to a PVD or CVD process, for example. A nitrogen-containing compound coats and/or penetrates the surface of the piston rings. Stainless steel has long been a preferred metal for piston rings as it is highly corrosive resistant and hard. Typically, stainless steel is about 81% iron, and about 18% chromium, with other alloying elements such as carbon, and nickel.
Because stainless steel is not typically highly alloyed, there are depassivation problems associated with a nitriding process, particularly involving a gas nitriding process. Efforts have been made to overcome depassivation problems such as raising the temperatures during nitriding, and prolonging the nitriding cycle. These steps are inefficient and therefore costly, with only limited success. Furthermore, stainless steel is rather difficult to prepare for machining of the piston rings. Coiling stainless steel wire requires a great amount of effort and time.
There is therefore a great need for a piston ring that is made from a strong, corrosion resistant metal that is easily and cheaply nitrided. There is also a need for a more efficient method for manufacturing piston rings.
The present invention relater to a piston ring that is broadly defined as a ring-shaped, non-stainless steel, iron alloy, sized and formed to accommodate an engine piston. The piston ring is nitrided, and the non-stainless steel enables a nitriding process to be performed in a relatively short interval, at lower temperatures than are required for stainless steel piston rings. The nitriding is most preferably performed using an ionic nitriding process.
The alloy includes between about 1% and about 10% chromium by weight, and preferably includes between about 1% and about 2% chromium by weight. By describing the alloy as a non-stainless steel, iron alloy, it is generally meant that the majority of the alloy is iron instead of, for example, titanium, and that chromium is present at a lower concentration than in stainless steel.
In a preferred embodiment of the invention, additional alloying elements are included. The alloy can further include carbon, manganese, and silicon, for example. Most preferably, chromium is included at no greater than 2% by weight, and carbon, manganese, and silicon are included at concentrations of no greater than about 1% by weight.
The alloy preferably includes even more alloying elements. One embodiment of the invention further includes phosphorus, sulfur, molybdenum, and vanadium, each being present at no greater than about 1% by weight. Another embodiment of the invention further includes aluminum and nickel, instead of phosphorus, sulfur, molybdenum, and vanadium. The aluminum and nickel are each included at no greater than about 1% by weight.
A particular embodiment of the invention includes a non-stainless steel, iron alloy that includes chromium at about 1.4% by weight, manganese at about 1% by weight, molybdenum at about 0.9% by weight, vanadium at about 0.2% by weight, carbon at about 0.2% by weight, silicon at about 0.1% by weight, phosphorus at about 0.02% by weight, and sulfur at about 0.004% by weight. Another particular embodiment of the invention includes a non-stainless steel, iron alloy that includes chromium at about 1.8% by weight, aluminum at about 1% by weight, nickel at about 1% by weight, manganese at about 0.7% by weight, carbon at about 0.3% by weight, and silicon at about 0.3% by weight.
Additional, advantages and novel features of the invention are set forth in the description that follows or may be learned by those skilled in the art through reading these materials or practicing the invention.